US 2009/0141124 A1 relates to a system and method for infrared video-based vital sign measurement using subject alignment, signal enhancement and harmonic analysis. In the method, contour segmentation is first performed to locate a blood vessel to be measured. Then, the same set of segmentation parameters is used in all frames to segment the corresponding region. After that, motion compensation is performed by aligning all pixels in the selected region, across frames based on the segmentations position and scale in each frame. After the contour segmentation and motion compensation, spatial filtering is performed to remove noise not related to heart beat, and non-linear filtering is then performed to process the temporal, signal corresponding to each aligned pixel sequence. The signal spectrum of each pixel is then fed to a clustering algorithm for outlier removal. Pixels in the largest cluster are then used to select the dominant frequency, and the median of the dominant frequency is output as the pulse rate.
A problem of the known method is that the outlier removal does not function well when the alignment of the segments has not been carried out accurately enough. In certain situations, particularly where the differences between corresponding pixels are at or below the quantization error of the pixel values, alignment is not accurate enough.